Well, there are a few things we can try, since it works fine on the other v126.96.36.199 installs I've tested.
What browser are you using? Version? Are other users experiencing the same problems? What browser and version are they using?
1. Have you cleared your cookies from your browser for your site?
2. Have you cleared the cache on your forum?
3. Could you go and edit the forum and save the page with the show all threads option selected, and try again.
5. Is all of your 3rd party code up to date? Providers commonly update their coding over the years and older coding can break over the years; providers such as google adsense, google analytics, ebay, amazon, etc.
Also, your link isn't applicable at all to anything recent; it's for UBB.Classic v6, which was in Perl, and was coded by a completely different developer (Charles Capps).